Some black soot in the exhaust may be normal if you idle a lot. When just sitting on the ground try and keep idle up to at least 2k rpm. Excessive black soot could be a sign of burning too rich and or running too much at low rpms or burning some oil. If you have no evidence of using oil (not likely) then make sure your cruise rpm is up in the 5K rpm range. Many seem to be right around 5100-5400 rpm. I'm usually around 5200 rpm. An over pitched prop could add to your black exhaust problem. Make sure the spark plug gap is within specs, .023-.027 for the gap. Burning too rich in fuel can be from high elevation and altitude operations, the needle clip in the carbs are in the wrong slot.
These are a few common causes that you should check.
Like I said some black soot may be normal.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
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